1. Field of the Invention
The invention relates to an interconnection element suitable for effective pressure connections between electronic components.
2. Description of Related Art
Interconnection or contact elements may be used to connect devices of an electronic component or one electronic component to another electronic component. For example, an interconnection element may be used to connect two circuits of an integrated circuit chip or to connect an application specific integrated circuit (ASIC) to another component such as a printed circuit board. Interconnection elements may also be used to connect the integrated circuit chip to a chip package suitable for mounting on a printed circuit board of a computer or other electronic device. Interconnection elements may further be used to connect the integrated circuit chip to a test device such as a probe card assembly or other printed circuit board (PCB) to test the chip.
Generally, interconnection or contact elements between electronic components can be classified into at least the two broad categories of xe2x80x9crelatively permanentxe2x80x9d and xe2x80x9creadily demountable.xe2x80x9d
An example of a xe2x80x9crelatively permanentxe2x80x9d contact element is a wire bond. Once two electronic components are connected to one another by a bonding of an interconnection element to each electronic component, a process of unbending must be used to separate the components. A wire bond interconnection element, such as between an integrated circuit chip or die and inner leads of a chip or package (or inner ends of lead frame fingers) typically utilizes a xe2x80x9crelatively permanentxe2x80x9d interconnection element.
One example of a xe2x80x9creadily demountablexe2x80x9d interconnection element is the interconnection element between rigid pins of one electronic component received by resilient socket elements of another electronic component, for example, a spring-loaded LGA socket or a zero-insertion force socket. A second type of a xe2x80x9creadily demountablexe2x80x9d interconnection element is an interconnection element that itself is resilient or spring-like or mounted in or on a spring or resilient medium. An example of such an interconnection element is a tungsten needle of a probe card. The interconnection element of a probe card is typically intended to effect a temporary pressure connection between an electronic component to which the interconnection element is mounted and terminals of a second electronic component, such as a semiconductor device under test.
With regard to spring interconnection elements, generally, a minimum contact force is desired to effect reliable pressure connection to an electronic component (e.g., to terminals of an electronic component). For example, a contact (load) force of approximately 15 grams (including as little as 2 grams or less and as much as 150 grams or more, per terminal) may be desired to effect a reliable electrical pressure connection to a terminal of an electronic component.
A second factor of interest with regard to spring interconnection elements is the shape and metallurgy of the portion of the interconnection element making pressure connection to the terminal of the electronic component. With respect to the tungsten needle as a spring interconnection element, for example, the contact end is limited by the metallurgy of the element (i.e., tungsten) and, as the tungsten needle is made smaller in diameter, it becomes commensurately more difficult to control or establish a desired shape at the contact end.
In certain instances, spring interconnection elements themselves are not resilient, but rather are supported by a resilient membrane. Membrane probes exemplify this situation, where a plurality of microbumps are disposed on a resilient membrane. Again, the technology required to manufacture such contact elements limits the design choices for the shape and metallurgy of the contact portion of the contact elements.
Commonly-owned U.S. patent application Ser. No. 09/152,812 filed Nov. 16, 1993 (now U.S. Pat. No. 5,476,211, issued Dec. 19, 1995), and its counterpart commonly-assigned divisional U.S. patent application Ser. No. 09/397,779, filed Sept. 16, 1999, titled xe2x80x9cElectronic Assembly Comprising a Substrate and a Plurality of Springable Interconnection Elements Secured to Terminals of the Substrate,xe2x80x9d (now U.S. Pat. No. 6,252,175, issued Jun. 26, 2001 and U.S. patent application Ser.No. 09/245,499, filed Feb. 5, 1999, by Khandros, titled xe2x80x9cMethod of Manufacturing Raised Electrical Contact Pattern of Controlled Geometry,xe2x80x9d disclose methods for making spring interconnection elements. In a preferred embodiment, these spring interconnection elements, which are particularly useful for micro-electronic applications, involve mounting an end of a flexible elongate element (e.g., wire xe2x80x9cstemxe2x80x9d or xe2x80x9cskeletonxe2x80x9d) to a terminal on an electronic component, and coating the flexible element and adjacent surface of the terminal with a xe2x80x9cshellxe2x80x9d of one or more materials. One of skill in the art can select a combination of thickness, yield strength, and elastic modulus of the flexible element and shell materials to provide satisfactory force-to-deflection characteristics of the resulting spring interconnection elements. Exemplary materials for the core element include gold. Exemplary materials for the coating include nickel and its alloys. The resulting spring interconnection element is suitably used to effect pressure, or demountable, interconnections between two or more electronic components, including semiconductor devices.
As electronic components get increasingly smaller and the spacing between terminals on the electronic components get increasingly tighter or the pitch gets increasingly finer, it becomes increasingly more difficult to fabricate interconnections including spring interconnection elements suitable for making electrical connection to terminals of an electronic component. Co-pending and commonly-assigned U.S. patent application Ser. No. 08/802,054, filed Feb. 18, 1997 (now U.S. Pat. No. 6,482,013, issued Nov. 19, 2002) discloses a method of making spring interconnection elements through lithographic techniques. In one embodiment, that application discloses forming a spring interconnection element (including a spring interconnection element that is a cantilever beam) on a sacrificial substrate and then transferring and mounting the interconnection element to a terminal on an electronic component. In that disclosure, the spring interconnection element is formed in the substrate itself through etching techniques.
In co-depending, commonly-assigned U.S. patent application Ser. No.08/852,152 filed May 6, 1997 (now U.S. Pat. No. 6,184,053), titled xe2x80x9cMethod of Making Microelectronic Spring Contact Elements,xe2x80x9d spring interconnection elements are formed on a substrate, including a substrate that is an electronic component, by depositing and patterning a plurality of masking layers to form an opening corresponding to a shape embodied for the spring interconnection element, depositing cinductive material in the opening made by the patterned masking layers, and removing the masking layers to form the free-standing spring interconnection element.
Co-pending and commonly-assigned U.S. patent application Ser. No. 09/023,859, titled xe2x80x9cMicroelectronic Contact Structures and Methods of Making Same,xe2x80x9d describes an interconnection element having a base end portion (post component), a body portion (beam component) and a contact end portion (tip component) and methods separately forming each portion and joining the post portion together as desired on an electronic component.
U.S. Pat. No. 5,613,861 (and its counterpart divisional U.S. Pat. No. 5,848,685), issued to Smith et al. disclose photolithographically patterned spring interconnection elements formed on a substrate with a body having an inherent stress gradient formed of a resilient (e.g., elastic) material such as chrome-molybdenum alloy or a nickel-zirconium alloy. The stress gradient causes an end of the body to bend away from the substrate in the shape of an arc when the end is freed from the substrate.
In order to achieve the desired shape of the body, the thickness of the interconnection element described in U.S. Pat. No. 5,613,861 must be limited. A limit on the thickness of the interconnection element limits the spring constant, k, of the interconnection element (kxe2x88x9d thickness), particularly in state-of-the-art interconnection element arrays where the dimensions (e.g., length and width) of individual interconnection arrays are reduced to accommodate a corresponding increase in contact pad or terminal density. A reduction of the spring constant generally reduces the amount of load or force, F, that may be applied to resilient interconnection elements for a given deflection, x (k=F/x). Thus, such interconnection elements generally cannot sustain the minimum contact force necessary to effect reliable pressure contact to an electronic component.
What is needed is a resilient interconnection element and a method of improving the resiliency of an interconnection element, particularly interconnection elements that are suitable for present fine-pitch electrical connections and that is/are scalable for future technologies. Also needed are improved methods of making resilient interconnection elements, particularly methods that are repeatable, consistent, and inexpensive.
An interconnection element is disclosed. In one embodiment, the interconnection element includes a first structure of a first material having a first spring constant and a second structure of a second material coupled to the first material. The first structure is capable of being free-standing by itself and the first spring constant is high enough for repeated elastic displacement without substantial plastic deformation. The second structure can be of lithographically-patterned second material. Collectively, the first material and the second material have a spring constant greater than the first spring constant. In one embodiment, the interconnection element is adapted to be coupled to an electronic component to act as a conductive path for the electronic component.
According to the invention, the spring constant of an interconnection element can be increased by coupling (e.g., coating) a second material over the first material. The first material is, for example, a body of the interconnection element formed to have some measurable amount of resiliency. The interconnection element of the invention increases this resiliency by coupling a second material that itself comprises resilient properties. The second material may also be used to increase the spring constant of a resilient interconnection. An increased spring constant permits the interconnection element of the invention to sustain a desired contact force to effect reliable pressure contact to an electronic component, making the interconnection element suitable for use in a variety of applications, including as part of a densely-packed array of interconnection elements on an electronic component to couple to a corresponding array of contact pads or terminals on a second electronic component. The addition of the second structure to the first structure will generally increase the working force for a given deflection, but may decrease the deflection required to permanently deform the interconnection element. The addition of the second structure may also increase or decrease the longevity of the interconnection element for a number of compression cycles to failure of the interconnection element, depending on the material properties of the structure material. In general, the ratios of the yield stress, "sgr", and elastic modulus, E, of the two structures along with the thickness of the structures (and the total thickness of the interconnection element) will determine the final outcome.
A method is also disclosed. In one embodiment, the method includes forming a first structure coupled to a substrate, the first structure comprising an internal stress to define a shape suitable as an interconnection in an integrated circuit environment and coupling, such as by coating, a second structure to the first structure to form an interconnection element. Collectively, the first structure and the second structure have a strength greater than a strength of the first structure material when used alone as an interconnection element.
The method of the invention addresses, in one aspect, enhancing the spring constant of a resilient interconnection element to make the interconnection element suitable for use in an integrated circuit environment to act as a conductive path from an electronic component. By coupling (e.g., coating) a second structure to the first structure to improve the spring constant of an interconnection element, the method of the invention addresses the limitations of the interconnection elements formed in certain disclosures presented in the prior art.